1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to an organic thin-film transistor (OTFT) fabricated using a fluropolymer that is used as a bank for the purposes of containment and crystallization.
2. Description of the Related Art
As noted in Wikipedia, an organic field-effect transistor (OFET) is a transistor that uses an organic semiconductor in its channel. OTFTs are a type of OFET. OTFTs can be prepared either by vacuum evaporation of small molecules, by solution-casting of polymers or small molecules, or by mechanical transfer of a peeled single-crystalline organic layer onto a substrate. These devices have been developed to realize low-cost, large-area electronic products. OTFTs have been fabricated with various device geometries.
Organic polymers, such as poly(methyl-methacrylate) (PMMA), CYTOP, PVA, polystyrene, parylene, etc., can be used as a dielectric. OFETs employing numerous aromatic and conjugated materials as the active semiconducting layer have been reported, including small molecules such as rubrene, tetracene, pentacene, diindenoperylene, perylenediimides, tetracyanoquinodimethane (TCNQ), and polymers such as polythiophenes (especially poly 3-hexylthiophene (P3HT)), polyfluorene, polydiacetylene, poly 2,5-thienylene vinylene, poly p-phenylene vinylene (PPV). These can be deposited via vacuum or solution base methods, the later being of interest for printed electronics. The newer generation of solution processable organic semiconductors consists of blends of high performance small molecule and polymeric molecules for optimum performance and uniformity.
FIG. 8 is a plan view photograph of an OSC ink material deposited without the benefit of containment banks (prior art). A specific example of an organic semiconductor material in a bottom gate OTFT is shown. Since the OSC print does not have any bank control, edge pinning and nucleation of the grains from the edges result, leading to varying grain size and non-uniform grain growth in the channel. In addition to the containment of the OSC ink, it is also important to have some control over the crystallization in this layer as the solvent evaporates. Lack of OSC containment leads to the inability to form orthogonal geometries due to surface tension dynamics. Typically, a deposited OSC ink begins drying first at the print area edges and the OSC grains nucleate from these edges. As a result, a roughly circular print area is obtained, regardless of the target print geometry, where the crystallization starts at the edges and proceeds inwards in the print area. Since more solvent is driven to the edges, higher grain sizes are obtained at the edges with increasing non-uniformity closer to the center. Such non-uniform and unpredictable grain growth is highly undesirable.
In most formulations the solvents being used are volatile enough that they start drying immediately after printing, before the anneal step. This drying leads to what is typically termed as a “coffee stain” effect in case of inkjet printing resulting from edge pinning and preferential drying at the print edges. This effect causes a solvent flow from the interior regions of the print area to the edges (convective flow). There are some examples in the literature showing that the addition of certain solvents can reverse this flow (termed Marangoni flow) to some extent. In the case of polymeric systems, which mostly form amorphous films, the coffee stain effect only leads to variations in the thickness of the film from edge to center. However in case of small molecule systems, which are crystallized to form polycrystalline films, there is added complication of the nucleation and grain growth in the film that has to be controlled. Since the print edges tend to dry first, there is a spontaneous tendency for grain nucleation at the edges. As further material is drawn from the interior regions of the print area, the grain growth proceeds towards the center of the print area.
This situation poses two main problems. If the surface tension of the substrate forces the formulation into a large nearly circular geometry (as shown), then it is not possible to print very small OSC features. If the volume dispensed is spread over a large area, then the large spread leads to inadequate volume for large grain growth through the entire print area—leading to small grains in the middle of the print area where the OTFT channel is defined.
In the case of printed small molecule OTFTs, the morphology and patterning of the organic semiconductor (OSC) layer is a challenging problem. Two key areas of research involve optimization of the grain growth in the OTFT channel region and isolation of the channel region of the device from the surrounding areas. Organic printed electronics rely on the ability to solution process and/or print each of the TFT layers. Inkjet (IJ) printing is commonly used to print the organic semiconductor layer. For a successful IJ print of the OSC layer, the solution and the surface energy must be optimized over a large area to control the extent of the OSC drop spread and uniformity of the drop. This is essential to insure the consequent morphology is as desired and consistent from device to device. A common way of addressing this issue has been to use so-called bank structures—which is an additional layer that is deposited and patterned in order to create a well structure. The organic semiconductor material is contained by jetted material only into a raised moat region of the coffee stain. The bank material restricts the drop spread and also helps maintain the solution uniformly over the well area. As a result, the process allows for a consistent film thickness and morphology uniformity with controlled drop drying.
The printing community has been using different methods of fabricating bank structures. The concept of using fluropolymer materials as an aid in fabrication has been explored to some extent in previous works. For example, in U.S. Pat. No. 6,838,361, a printed hydrophobic layer is used (with fluropolymer as example of such layer) to create a separation between two printed metal lines, but not necessarily to contain the metal print within certain area. These two metal lines, separated by the fluropolymer, then serve as the source/drain electrodes for a TFT. However, in order for this concept to work, the bank layer must be removed after the metal layer deposition using a plasma process. This removal process puts limitations on the types of material surfaces that can be used. Especially in the case of a bottom gate organic TFT, any plasma step used to remove the bank layer can have detrimental effect on the underlying organic gate insulator layer. The plasma step can also oxidize or otherwise damage the metal S/D interfaces.
Another method using fluropolymers is the deposition of a blanket layer (or dual layers), with patterning using standard lithographic techniques, see US 2007/0193978A1, WO2009077738A1, WO2010020790A1, and WO2009112569A1. The challenge to these approaches is to ensure that the bank material develops cleanly over the organic gate insulator and the metal source drain electrodes, while still not causing any damage to these structures in the development process.
Gundlach et al., “Contact-Induced Crystallinity for High-Performance Soluble Acene-Based Transistors and Circuits”, use a different approach to address the containment issue. In that work they preferentially coat the S/D electrodes with self-assembled monolayers and then blanket coat an OSC film. Large OSC grain growth nucleates only on these electrodes and then bridges the channel for high performance OTFTs. However, poor grain growth outside the channel area is used as a means to provide for good device isolation. This concept works only to a limited extent, with off currents>nA (nanoamps), and is not reasonable for scaling and application in practical products.
It would be advantageous if a banking structure could be deposited using an inkjet process, to contain OSC deposition, without the requirement of lithographically patterning the bank.
It would be advantageous if the above-mentioned banking structure could be left in place after device fabrication.
It would be advantageous if the above-mentioned banking structure aided in the crystallization of the deposited OSC material.